While millions of birds worldwide also die in this way, many studies have been done to mitigate the problem overseas. The society, however, noted that such a phenomenon is "chronically understudied" in Asia.

With the avian migratory season under way, the Nature Society's Bird Group has started a survey and is asking for those who have witnessed dead or injured migratory birds here to come forward.

It plans to collect information from now until next May and release a preliminary report by late next year. There are plans to run the survey for at least five years to observe short-term trends.

Mr Yong Ding Li, 30, a coordinator of the project, said such crashes may lead to a loss in the bird population, which is already in decline due to habitat loss, hunting and climate change.

"If we know which species are more affected, and what settings increase their risk of crashes, we might then be able to make recommendations to mitigate this," said Mr Yong, a graduate student specialising in ecology and bird conservation at the Australian National University.

Each year, some 2,000 migratory birds from countries like Russia, Mongolia, China and Japan arrive in Sungei Buloh in August and September for a respite from harsh winters, said the National Parks Board.

Some fly off again, heading to Australia or Indonesia, while others live in Sungei Buloh and surrounding areas till the next March or April.

Mr Yong explained that migratory birds crash into buildings because many fly at night. They are often attracted to, or disoriented by, the lighting from buildings, as they navigate using star patterns of the night sky.

They may also be confused by the reflection of trees and sky on the buildings' exterior.

Strix Wildlife Consultancy director Subaraj Rajathurai, 51, noting that the study was interesting and worthwhile, said: "We know this is happening but we don't know on what scale. "But it's not an easy study to do because we have such an efficient clean-up system in Singapore... Our clean-up crew may sweep away the dead birds before anyone wakes up."

Bird Ecology Study Group co-founder Wee Yeow Chin, 77, said: "In other countries, there are architectural adaptations so that birds don't crash. This study can help us find out the extent of the tragedy and whether we need to take some steps to crack down on this."